This invention relates to a picture printing control method employed in printing a picture with negative color film whose quality has changed due to a lapse of time or unsuitable storage.
When a sensitized material is stored for a considerably long period, or if it is stored in unsuitable circumstances, the sensitivity of at least one of the sensitized layers thereof will be decreased or increased, i.e., the characteristic curve thereof is changed. If a color negative film whose characteristic has been changed as described above is used to print a picture on color paper under the same exposure conditions as a normal negative, the resultant print is considerably non-uniform in chromatic balance.
Negative color films produced by amateur cameramen are often photographed under various light sources such as daylight or light from tungsten or fluorescent lamps and the object images in the films are biased variously in hue. Therefore, heretofore, in automatic exposure correction using a print making apparatus, the exposure is, in general, corrected as follows:
Correction of the deviation in chromatic balance due to the light source used is carried out according to Evans's principle (cf. U.S. Pat. No. 2,571,697). When an ordinary object is photographed, the percentages of the three color components, blue, green and red are substantially equal to one another as a whole, and therefore if the light passed through the negative is integrated and mixed with respect to the entire image, then a substantially constant chromatic balance can be obtained in correspondence to the light source used for photography. It is desirable for a print that the correction is made so that the percentages of the three color components are substantially equal over the entire image. Therefore, if the average transmission density of the entire negative image, i.e., a large area average density (LATD) is represented by Di (where i represents any one of the blue, green and red color components), the exposure time Ti for each color may be obtained from the following expression: EQU log T.sub.i =.alpha.i Di+.beta.i
where .alpha. and .beta. are constants.
Therefore, the exposure time is decreased when the large area average density Di of a color is small, while the exposure time is increased when the value Di is large, to correct the density and the chromatic balance.
Even when the same light source is used in taking pictures, sometimes the hue deviates greatly depending on the objects photographed. For instance in the case where a person in front of a green lawn or a red wall is photographed, the proportion of the three color components blue, green and red deviates greatly as a whole (this is known as "color failure"). In the case when color correction is applied to the negative object according to Evans's principle, the face of the person photographed with the green background appears rather magenta in the print, and the face of the person photographed with the red background appears rather cyan in the print; that is, the color of the resultant print does not agree with the color of the photographed object. If, in the case where such an object is photographed in daylight, the average value D of the values Di of the three colors is inserted for Di in the equation for calculating the exposure times Ti described above, a print suitable in charomatic balance can be obtained without being affected by the deviation in hue of the object. Accordingly, if the correct chromatic balance can be obtained with the exposure times for an ordinary object in daylight being the same for all three colors, a print correct in chromatic balance can be obtained for an object photographed with a green or red background by making the exposure times for the three colors equal (the exposure times being increased or decreased, as a whole, according to the average density) irrespective of the object.
Roughly stated, there are two methods of correcting chromatic balance, as is apparent from the above description, which are called "high correction" and "lower correction", respectively. The "high correction" is effective in correcting the deviation in chromatic balance due to the light source used, and the "lower correction" is effective in correcting for color failure, as described above.
The "high correction" and the "lower correction" methods are in contradiction with each other. In practice, the negatives handled in a developing laboratory are of objects photographed under various light sources and include various color failures. Therefore, the degree of correction in the picture printing apparatus is, in general, set somewhere between "high correction" and "lower correction". The "high correction" or the "lower correction" is selected depending upon the content of the images in the negative color film (i.e., depending upon the light sources used in photography and the deviations in hue of the objects photographed), or correction keys ae suitably selected, so that the exposure times for three colors are changed independently in balance, thereby to correct the chromatic balance of negatives which cannot be corrected by the ordinary setting.
Negatives which cannot be corrected by the ordinary setting as described above include those have been changed in quality or characteristic because of a lapse of time, those which have been used in photographing objects in light from a tungsten or fluorescent lamp, and those which include serious color failures. In order to automatically discriminate such negatives, a method is employed in which the image in a color negative is subjected to three-color (red, green and blue) separation scanning, to detect local due information in the image, so that calculation may be effected according to the hue information thus detected.
However, in the case of a negative whose quality has changed because of a lapse of time or insufficient storage, it is difficult to sufficiently detect the degree of quality change by the method in which the image in the negative is subjected to three-color separation scanning to detect local hue information in the image, as described above, because the degree of quality change in the negative is not uniform, and mixes with the hue of the object itself.